U.S. patent application number 10/489161 was filed with the patent office on 2005-01-13 for process for improving the purity of quaternary ammonium hydroxides by electrolysis in a two-compartment cell.
Invention is credited to Boerman, Gerrit Jan, Korpel, Fred, Rains, Roger Keranen.
Application Number | 20050006252 10/489161 |
Document ID | / |
Family ID | 8181052 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006252 |
Kind Code |
A1 |
Korpel, Fred ; et
al. |
January 13, 2005 |
Process for improving the purity of quaternary ammonium hydroxides
by electrolysis in a two-compartment cell
Abstract
The present invention relates to a process for purifying a
recycle base solution waste stream of a composition comprising a
quaternary ammonium hydroxide comprising the steps of (a) providing
an electrolysis cell which comprises an anolyte compartment
containing an anode, a catholyte compartment containing a cathode,
and a cation-selective membrane separating the anolyte and
catholyte compartments, (b) charging the recycle base solution
waste stream comprising the quaternary ammonium hydroxide to be
purified to the anolyte compartment and charging water, optionally
containing a quaternary ammonium hydroxide, to the catholyte
compartment, (c) passing a current through the electrolysis cell to
produce a purified aqueous quaternary ammonium hydroxide solution
in the catholyte compartment, (d) recovering the purified aqueous
quaternary ammonium hydroxide solution from the catholyte
compartment, (e) washing the anolyte compartment with a suitable
solvent, and (f) repeating steps (b)-(e). The process is
particularly suitable for improving the purity of an aqueous
solution comprising tetramethyl ammonium hydroxide, which was used
in the production of 4-aminodiphenylamine for a number of reaction
cycles.
Inventors: |
Korpel, Fred; (Roosendaal,
NL) ; Boerman, Gerrit Jan; (Deventer, NL) ;
Rains, Roger Keranen; (Richfield, OH) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Family ID: |
8181052 |
Appl. No.: |
10/489161 |
Filed: |
May 7, 2004 |
PCT Filed: |
October 7, 2002 |
PCT NO: |
PCT/EP02/11454 |
Current U.S.
Class: |
205/746 |
Current CPC
Class: |
C07C 209/84 20130101;
C07C 209/84 20130101; C07C 211/63 20130101; C25B 3/00 20130101 |
Class at
Publication: |
205/746 |
International
Class: |
B01J 002/00; B01D
059/50; C30B 009/14 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2001 |
EP |
01203863.4 |
Claims
1. A process for purifying a recycle base solution waste stream of
a composition comprising a quaternary ammonium hydroxide comprising
the steps of (a) providing an electrolysis cell which comprises an
anolyte compartment containing an anode, a catholyte compartment
containing a cathode, and a cation-selective membrane separating
the anolyte and catholyte compartments, (b) charging the recycle
base solution waste stream comprising the quaternary ammonium
hydroxide to be purified to the anolyte compartment and charging
water, optionally containing a quaternary ammonium hydroxide, to
the catholyte compartment, (c) passing a current through the
electrolysis cell to produce a purified aqueous quaternary ammonium
hydroxide solution in the catholyte compartment, (d) recovering the
purified aqueous quaternary ammonium hydroxide solution from the
catholyte compartment, (e) washing the anolyte compartment with a
suitable solvent, and (f) repeating steps (b)-(e).
2. The process according to claim 1 wherein the anolyte compartment
is charged with an aqueous solution comprising tetramethyl ammonium
hydroxide (TMAH).
3. The process according to claim 1 or wherein the anolyte
compartment is charged with an aqueous solution containing 5 to 40
wt. % of TMAH.
4. The process according to claim 1 wherein the anolyte compartment
is charged with an aqueous solution comprising TMAH, which was used
in the production of 4-aminodiphenylamine for a number of reaction
cycles, and optionally aniline.
5. The process according to claim 1 wherein the catholyte
compartment is charged with an aqueous solution of a quaternary
ammonium hydroxide, which is the same as the quaternary ammonium
hydroxide present in the composition to be purified.
6. The process according to claim 5 wherein the catholyte
compartment is charged with an aqueous 5 to 25 wt. % TMAH
solution.
7. The process according to claim 1 wherein the electrolysis is
stopped once a pH of 1 to 7, preferably 4 to 7, is reached in the
anolyte compartment.
8. The process according to claim 1 wherein the cation-selective
membrane is a perfluorinated membrane.
9. The process according to claim 1 wherein the solvent is
aniline.
10. The process according to claim 1 wherein between steps e) and
f) the anolyte compartment is washed with water.
11. The process according to claim 1 wherein the recycle base
solution waste stream comprises a quaternary ammonium hydroxide and
a quantity of a quaternary ammonium salt or a mixture of quaternary
ammonium salts that can be converted to the quaternary ammonium
hydroxide by electrolysis.
12. The process according to claim 11 wherein the recycle base
solution waste stream comprises TMAH and at least one of
tetramethal ammonium carbonate and tetramethyl ammonium hydrogen
carbonate.
Description
[0001] The present invention relates to a process for improving the
purity of a recycle base solution waste stream comprising a
quaternary ammonium hydroxide.
[0002] Quaternary ammonium hydroxides such as tetramethyl ammonium
hydroxide (TMAH) are used inter alia as a developer for
photoresists in the manufacture of printed circuit boards and
microelectronic chips and as a base in the production of
4-aminodiphenylamine (4-ADPA). Alkylated derivatives of 4-ADPA such
as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene- diamine (6-PPD) are
used as antidegradants in rubber compositions and rubber articles
such as tires.
[0003] In said production of 4-ADPA, the base--which typically is
in the form of an aqueous solution--is recycled many times
(hereinafter also referred to as recycle base solution). However,
after a certain number of reaction cycles, the active content of
the aqueous base solution has decreased to such an extent that it
can no longer be used in the production process and either some of
the recycle aqueous base solution is purged and replaced with fresh
waste solution or all of it is discarded as waste, which adds to
the cost of the 4-ADPA and the 6-PPD prepared thereof. The present
invention provides a solution to this waste problem. Also, with an
increasing number of reaction cycles, the liquid-liquid separation
of the aqueous waste solution from the 4-ADPA-containing organic
phase proceeds with greater difficulty.
[0004] When TMAH is used as the base, the purged/discarded aqueous
recycle base solution contains inter alia various tetramethyl
ammonium (TMA) salts, such as tetramethyl ammonium acetate,
formate, chloride, carbonate, and oxalate as well as aniline--one
of the starting materials for preparing 4-ADPA. It further contains
small amounts of various other salts and other organic impurities.
Quaternary ammonium hydroxides are typically prepared by means of
electrolysis. For example, TMAH may be prepared from tetramethyl
ammonium chloride using a two-compartment electrolysis cell
comprising an anolyte compartment containing an anode and a
catholyte compartment containing a cathode, said compartments being
separated by a cation-selective membrane. Said membrane is also
referred to in the art as a cation-exchange membrane. In this
manufacturing process, the quaternary ammonium salt from which the
quaternary ammonium hydroxide is prepared is charged to the anolyte
compartment. A method of manufacturing TMAH was disclosed in U.S.
Pat. No. 4,572,769. This method describes the synthesis of TMAH
from tetramethyl ammonium formate by electrolysis, however, not the
purification of TMAH from a recycle base solution. In U.S. Pat. No.
4,394,226 similarly, TMAH is prepared from tetramethyl ammonium
halide, particularly chloride, in an electrolytic cell, but no
purification of a recycle base solution containing TMAH was
disclosed.
[0005] It is also known in the art to improve the purity of
mixtures comprising a quaternary ammonium hydroxide by
electrolysis.
[0006] For example, U.S. Pat. No. 4,714,530 discloses a process for
producing high-purity quaternary ammonium hydroxides by means of
electrolysis using a two-compartment electrolysis cell equipped
with a cation-exchange membrane in which an aqueous solution
containing the quaternary ammonium hydroxide is charged to the
anolyte compartment. This method does not relate to a recycle base
solution containing TMAH, but relates to improving the purity of
freshly prepared TMAH, which process, moreover, makes use of a
method leading to deposition of solid material at the anode.
[0007] We found that the electrolysis of the waste stream of
recycle TMAH--as obtained from the production of 4-ADPA--by
charging it to the anolyte compartment of a two-compartment
electrolysis cell soon after its start resulted in the formation of
a significant amount of a solid material at the anode, which fouled
the electrode and the anolyte compartment and virtually stopped the
electrolysis after some time (see Comparative Examples A and
B).
[0008] Surprisingly, we subsequently found that these problems were
less severe or even did not occur when the electrolysis was carried
out in accordance with the present invention.
[0009] The process for purifying a recycle base solution waste
stream of a composition comprising a quaternary ammonium hydroxide
in accordance with the present invention comprises the steps of
[0010] (a) providing an electrolysis cell which comprises an
anolyte compartment containing an anode, a catholyte compartment
containing a cathode, and a cation-selective membrane separating
the anolyte and catholyte compartments,
[0011] (b) charging the recycle base solution waste stream
comprising the quaternary ammonium hydroxide to be purified to the
anolyte compartment and charging water, optionally containing a
quaternary ammonium hydroxide, to the catholyte compartment,
[0012] (c) passing a current through the electrolysis cell to
produce a purified aqueous quaternary ammonium hydroxide solution
in the catholyte compartment,
[0013] (d) recovering the purified aqueous quaternary ammonium
hydroxide solution from the catholyte compartment,
[0014] (e) washing the anolyte compartment with a suitable solvent,
and
[0015] (f) repeating steps (b)-(e).
[0016] In the case of recycle base, which is obtained from the
production of 4-ADPA, the invention process results in the recovery
from the catholyte compartment of an aqueous solution containing
lower amounts of anions, such as acetate, formate, chloride,
carbonate, and oxalate, than are present in the recycle base and,
if desired, having a higher quaternary ammonium hydroxide content.
Typically, the recovered aqueous solution also contains a
portion/fraction of the neutral organic compounds such as aniline,
which are present in the recycle base.
[0017] Due to the fact that the anolyte and catholyte compartments
contain aqueous solutions, oxygen gas is formed at the anode and
hydrogen gas is formed at the cathode. The presence of tetramethyl
ammonium carbonate and/or tetramethyl ammonium bicarbonate in the
anolyte compartment may cause the formation of carbon dioxide gas,
which depends on the pH of the aqueous solution in the anolyte
compartment. These gases are handled and processed in a
conventional way.
[0018] The invention process can be carried out using any known
electrolysis cell equipped with conventional electrodes and
cation-selective membranes, provided said electrodes and membranes
are compatible with the solutions which are charged to and which
are formed in the anolyte and catholyte compartments.
[0019] The anode and the cathode may be made from a variety of
materials. The anode must be suitable for oxygen
formation/evolution and the cathode for hydrogen
formation/evolution. Suitable anodes and cathodes are known to a
person of ordinary skill in the art. The cathode may also be an
oxygen reducing/oxygen depolarized cathode. Preferably, a platinum
anode and a stainless steel cathode are used.
[0020] The cation-selective membrane may be any of those, which
have been used in the electrolysis of quaternary ammonium salts to
quaternary ammonium hydroxides and the electrolytic purification of
quaternary ammonium hydroxides. A variety of suitable
cation-selective membranes are available to a person of ordinary
skill in the art. A distinction is made between perfluorinated and
non-perfluorinated membranes. Preferably, the cation-selective
membrane to be used in accordance with the present invention is a
perfluorinated membrane, for example made from
polytetrafluoroethylene, such as the ones sold under the name
Nafion by DuPont. Other suitable cation-selective membranes include
membranes made from polyethylene, polypropylene, polyvinylchloride,
polystyrene-divinylbenzene, and (sulfonated) polysulfone.
[0021] Apart from the fact that cation-selective membranes allow
the passage of cations and prevent the transport of anions, .said
membranes are also selective for the type of cation. For example,
in the art proton-selective membranes are known. Preferably, the
invention process is carried out using a membrane selective for the
quaternary ammonium ion, which is present in the composition
comprising the quaternary ammonium hydroxide to be purified.
[0022] The quaternary ammonium hydroxide-containing compositions
which are purified in accordance with the process of the present
invention typically are aqueous solutions containing from 1 to 45,
preferably 5 to 40, more preferably 10 to 35 wt. % of quaternary
ammonium hydroxide. These compositions may contain an organic
solvent. They may also contain an inorganic hydroxide such as
sodium hydroxide, potassium hydroxide or cesium hydroxide.
[0023] The quaternary ammonium hydroxide-containing composition to
be used in the process of the present invention may contain any
quaternary ammonium hydroxide. Typically, the composition comprises
a tetrahydrocarbyl ammonium hydroxide or hydrocarbylene
di(trihydrocarbyl)ammonium dihydroxide. The composition may also
comprise a mixture of a quaternary ammonium hydroxide and an
inorganic hydroxide. Typical examples include tetramethyl ammonium
hydroxide, tetrapropyl ammonium hydroxide, tetrabutyl ammonium
hydroxide, choline hydroxide, phenyltrimethyl ammonium hydroxide,
benzyltrimethyl ammonium hydroxide, and
bis-(dibutylethyl)hexamethylene diammonium hydroxide (hexamethylene
1,6-bis(dibutylethyl)ammonium dihydroxide). Other suitable examples
have been described in the prior art cited above, i.e. U.S. Pat.
No. 4,714,530 (col. 2, I. 60 through col. 3,I. 2) and U.S. Pat. No.
5,389,211 (col. 5, II. 43-60). Preferably, the composition
comprises tetramethyl ammonium hydroxide (TMAH). More preferably,
the composition to be purified in accordance with the present
invention is an aqueous solution, which was used in the production
of 4-ADPA for a number of reaction cycles (i.e. recycle base waste
stream), most preferably, an aqueous solution comprising TMAH. The
recycle base waste stream typically contains aniline. The recycle
base waste stream may also contain an inorganic hydroxide.
[0024] The quaternary ammonium hydroxide-containing composition to
be used in the process of the present invention may also be a
mixture of a recycle base solution and other compounds, such as a
quantity of a suitable fresh quaternary ammonium salt or salt
mixture that can be converted to the quaternary ammonium hydroxide
by electrolysis concurrently with the electrolysis of salts in the
recycle base solution. For example, tetramethyl ammonium carbonate
and/or tetramethyl ammonium hydrogen carbonate can be added to an
aqueous recycle TMAH stream. Addition of a suitable fresh
quaternary ammonium salt or salt mixture to a recycle base
solution, for production of fresh quaternary ammonium hydroxide
concurrently with recovery of quaternary ammonium hydroxide from
salts in the recycle base, can also be conducted with an
electrolysis cell, which comprises an anolyte compartment
containing an anode, a catholyte compartment containing a cathode,
and at least one intermediate compartment, which is separated from
the anolyte and catholyte compartments by cation-selective
membranes.
[0025] At the start of electrolysis, the catholyte compartment
contains water, optionally containing a quaternary ammonium
hydroxide. Preferably, demineralized or soft water is used in the
invention process. The conductivity of the catholyte solution in
the catholyte compartment is increased by including a quaternary
ammonium hydroxide. The presence of electrolytes in the anolyte and
catholyte compartments allows current to flow through the
electrolysis cell immediately after the start of the electrolysis.
It is to be noted that it is not critical to the invention process
which electrolyte-containing aqueous solution is present in the
catholyte compartment. The choice will mainly be determined by the
desired purity and the desired active content of the aqueous
quaternary ammonium hydroxide solution to be recovered from the
catholyte compartment. Preferably, the desired active content is in
the range of 15 to 25 wt. %, more preferably about 20 wt. %.
[0026] Preferably, the catholyte compartment contains an aqueous
solution of a quaternary ammonium hydroxide, which is the same as
the quaternary ammonium hydroxide present in the composition to be
purified. A practical catholyte solution to start with is an
aqueous 1 to 35, preferably 5 to 25, more preferably 5 to 20 wt. %
solution of the quaternary ammonium hydroxide. Preferably, the
catholyte compartment is charged with an aqueous quaternary
ammonium hydroxide solution of high purity, e.g., a solution having
the desired purity. The active content may vary as desired. More
preferably, an aqueous TMAH solution is used as the starting
catholyte solution.
[0027] The invention process is operated either as a batch process
or as a semi-continuous process. It is practical to use a batch
process. Preferably, the invention process is carried out by
charging a batch of the composition comprising the quaternary
ammonium hydroxide to be purified to the anolyte compartment and
continuing the electrolysis until practically all of the quaternary
ammonium ions are removed therefrom before charging a subsequent
batch to the anolyte compartment. In the case of recycle base, it
was found to be advantageous to dilute the recycle base with water
before charging it to the anolyte compartment of the electrolysis
cell. The processed batch--present in the anolyte compartment--may
either be discarded wholly or partly and is then replaced by or
mixed with the subsequent batch, respectively. In the case of
recycle base, preferably a part of the processed batch--i.e. the
so-called heel--is mixed with a fresh portion of recycle base. More
preferably, about equal weight parts of heel and fresh recycle base
are charged to the anolyte compartment.
[0028] In accordance with the present invention, the anolyte
compartment of the electrolysis cell is washed with a suitable
solvent. It was found that solid material was formed in the anolyte
compartment during the electrolysis of a first batch of the
composition comprising the quaternary ammonium hydroxide to be
purified, in particular in the case of recycle base. As a result,
fouling of the electrode, the anolyte compartment, and the anolyte
compartment fluid circulation equipment, i.e. the circulation loop,
loop filter, circulation vessel, and circulation pump, occurred.
Suitable solvents are those, which dissolve the solid material that
is formed without affecting any part of the electrolysis equipment.
This can easily be determined by a person of ordinary skill in the
art. Suitable solvents include aniline and N,N-dimethylformamide,
N-methyl-2-pyrrolidone, and dimethyl sulfoxide. In the case of
recycle base, preferably aniline is used as the solvent. The
washing step is carried out at the end of the processing of each
batch (or semi-continuously processed batch) using as much of the
solvent as necessary. The amount of solvent to be used can easily
be determined by a person of ordinary skill in the art. Preferably,
after washing with a suitable solvent, the anolyte compartment is
washed with water before charging a new batch to the anolyte
compartment. In the case of recycle base and when using aniline as
the solvent, it is best to remove the aniline by washing with water
afterwards.
[0029] The solvent washing step typically is carried out at an
elevated temperature, preferably 40 to 80.degree. C., more
preferably 40 to 60.degree. C., most preferably 40 to 50.degree. C.
The washing with water typically is carried out at a temperature of
20 to 50.degree. C.
[0030] The electrolysis of the composition comprising the
quaternary ammonium hydroxide is effected by applying a direct
current between the anode and the cathode with a current density of
generally up to 4,000 A/m.sup.2. A practical range is from 500 to
1,500 A/m.sup.2. The current is applied to the electrolysis cell
for a period of time sufficient to allow for the transport of
preferably all quaternary ammonium ions from the anolyte
compartment to the catholyte compartment. An important parameter
for monitoring the progress of the invention process is the pH of
the aqueous solution in the anolyte compartment.
[0031] During the electrolysis of the composition comprising the
quaternary ammonium hydroxide to be purified, the pH of the
solution of the anolyte compartment decreases due to the generation
of protons in the anolyte compartment and the transport of
quaternary ammonium ions from the anolyte compartment to the
catholyte compartment. Anions such as chloride ions are unable to
pass the cation-selective membrane. A weak acid such as acetic
acid, however, is able to pass a cation-selective membrane by way
of diffusion. Preferably, the electrolysis is stopped once a pH of
1 to 7, more preferably 4 to 7, even more preferably 4 to 6, most
preferably about 5 is reached in the anolyte compartment. If only a
part of the processed composition--present in the anolyte
compartment--is replaced by a subsequent batch, the pH may be
maintained between certain chosen values, e.g., between 5 and
7.
[0032] Typically, the aqueous solutions present in the compartments
of the electrolysis cell are circulated by means of pumping in a
conventional way, for example, by using circulation loops,
circulation vessels, and pumps for each compartment separately.
These circulation loops may be provided with conventional
filters.
[0033] During the electrolysis, the temperature of the solutions
within the compartments typically is maintained at from 10 to
90.degree. C., preferably 40 to 80.degree. C., more preferably 40
to 60.degree. C, most preferably 40 to 50.degree. C.
[0034] The present invention is illustrated by the following
Examples.
COMPARATIVE EXAMPLES A AND B
[0035] Two one-batch experiments were performed using a
two-compartment Micro Flow Cell (from ElectroCell) comprising an
anolyte compartment containing an anode and a catholyte compartment
containing a cathode, said compartments being separated by means of
a cation-selective membrane. EPDM gaskets and Teflon frames were
used.
[0036] In the first experiment, i.e. Comparative Example A, a
Nafion 450 membrane (from DuPont) was used. The anode was a
platinum electrode, the cathode stainless steel (both from
ElectroCell). The recycle base was charged to the anolyte
compartment and it contained 13.61 wt. % TMAH. The starting
catholyte solution was 13.85 wt. % aqueous TMAH.
[0037] In the second experiment, i.e. Comparative Example B, a
Nafion 117 membrane was used. The anode was a DSA for oxygen
evolution, the cathode stainless steel (both from ElectroCell). The
recycle base was charged to the anolyte compartment and it
contained 12.68 wt. % TMAH, the catholyte was 12.09 wt. % aqueous
TMAH.
[0038] The results of these experiments are shown in Tables 1 to
3.
[0039] It was found that at the anode a significant amount of a
solid material was formed which fouled the electrode and had to be
removed periodically in order to be able to continue the
electrolysis. Ultimately, the electrolysis virtually stopped
(TMA.sup.+bound to carbonate was not transported from the anolyte
to the catholyte compartment). As a result, the electrolysis could
not be performed long enough for an economically attractive
recovery of TMAH to be obtained. In addition, the removal of this
solid was time consuming and cumbersome.
[0040] The detection limits are as follows: TMA-acetate (0.0023 wt.
%), TMA-formate (0.0013 wt. %), TMA-chloride (0.0015 wt. %),
TMA.sub.2-carbonate (0.0350 wt. %), TMA.sub.2-oxalate (0.0027 wt.
%), and TMAH (0.0100 wt. %).
1TABLE 1 Electrolysis data Comparative Example A B Average current
efficiency (%) 35 19 Average current density (A/m.sup.2) 1300 2400
Temperature (.degree. C.) 46 47 DC Voltage (V) 7.7 8.2
[0041]
2TABLE 2 Starting and recovered waste compositions Comp. Ex. A
A.sub.start A.sub.final C.sub.start C.sub.final TMA-Acetate wt. %
0.74 0.66 nm nm TMA-Formate wt. % 1.09 1.02 nm nm TMA-Chloride wt.
% 0.02 0.02 nm nm TMA.sub.2-Carbonate wt. % 12.08 18.18 0.16 0.32
TMA.sub.2-Oxalate wt. % 1.89 1.43 nm nm TMAH wt. % 13.61 0.33 13.85
21.98 Aniline wt. % 1.90 0.44 nd 0.24 Weight g 900 830 750 420
Water added g 100 Samples taken g 240 240
[0042] A.sub.start is the starting anolyte solution, A.sub.final is
the final anolyte solution, C.sub.start is the starting catholyte
solution, and C.sub.final is the final catholyte solution, nm means
not measurable (below the detection limit), nd means not
determined.
3TABLE 3 Starting and recovered waste compositions Comp. Ex. B
A.sub.start A.sub.final C.sub.start C.sub.final TMA-Acetate wt. %
0.67 1.08 nm nm TMA-Formate wt. % 1.06 1.25 nm nm TMA-Chloride wt.
% 0.02 0.02 nm nm TMA.sub.2-Carbonate wt. % 13.25 24.98 nm nm
TMA.sub.2-Oxalate wt. % 1.86 1.65 nm nm TMAH wt. % 12.68 nm 12.09
23.44 Aniline wt. % 1.79 0.56 nd 0.47 Weight g 900 570 750 530
Water added g 100 Samples taken g 120 120
[0043] A.sub.start is the starting anolyte solution, A.sub.final is
the final anolyte solution, C.sub.start is the starting catholyte
solution, and C.sub.final is the final catholyte solution, nm means
not measurable (below the detection limit), nd means not
determined.
EXAMPLE 1
[0044] A two-compartment Multi Purpose Cell (from ElectroCell)
equipped with a DSA anode, a stainless steel cathode, and a Nafion
324 (from Dupont) cation-selective membrane was operated, according
to a procedure similar to the procedure described in Comparative
Examples A and B (i.e. 12.5 V, 40-50.degree. C., final pH 5), with
6 batches of recycle base for a total (electrolysis) time of 64 h.
Each time, the composition to be electrolyzed consisted of a
mixture of 1,600 g of fresh recycle base, having a composition
similar to the compositions described in Comparative Examples A and
B, and 1,600 g of the so-called heel of the previously processed
batch of recycle base (i.e. each batch having a total weight of
3,200 g), and 700 g of the heel were discarded. At the end of the
processing of each batch, the purified aqueous TMAH solution was
recovered from the catholyte compartment and the anolyte.
compartment including the anolyte fluid circulation loop and
circulation vessel were emptied and the circulation vessel was
filled with 1,000 g of aniline. The aniline was circulated for 30
min through the anolyte compartment at 50.degree. C. Then, the
aniline wash was removed and the wash procedure was repeated with
1,000 g of water, which was circulated for 5 min at 20-50.degree.
C., the water being warmed up during circulation. After each
washing procedure, the next 3,200 g batch of recycle base plus heel
was charged to the anolyte compartment and subjected to
electrolysis.
[0045] The capacity of the electrolysis cell remained practically
unchanged, i.e. it was 40.31 moles TMA.sup.+/m.sup.2/h for the
first batch and 35.73 moles TMA.sup.+/m.sup.2/h for the sixth batch
(TMA.sup.+ stands for tetramethyl ammonium ion). Inspection of the
electrolysis cell after the processing of the 6 batches did not
show any fouling of the anode, the anolyte compartment or the
anolyte compartment fluid circulation equipment.
* * * * *